What is Beta 1, 3-D Glucan?

What is Beta 1, 3-D Glucan?
Beta 1, 3-D Glucan from Baker's Yeast (Saccharomyces cerevisiae) is a pharmacological agent that enhances resistance to infection.  Beta glucan has been shown to act through stimulation of immune cells. (Tsukagoshi, S. et al. 1984).

Glucans are long chain polysaccharide (sugar) molecules.  Polysaccharides represent a structurally diverse class of macromolecules and have a high capacity for carrying biological information because of their great structural variability (like DNA). 
Polysaccharides can interconnect at various points to form a wide range of branched or liner structures. 
In nature there are many forms of these glucans.
Beta glucan is many glucose molecules joined in a specific way with branches called beta 1,3 linkages.  These molecules have been shown to have specific stimulatory effects on the immune system.  source

The Immune Response Enhanced by Beta-1,3D Glucan - pg.2

The Immune Response Enhanced by Beta-1,3D Glucan by A.J.Lanigan - pg.2

As part of the continuing process, IL-1 helps activate B and T cells; IL-2 instructs other helper T's and a different class of T cells, the killer T's (CTLs or cytotoxic T lymphocytes), to multiply. The proliferating helper T's in turn release substances that cause B cells to multiply and produce antibodies. B cells are prepared to recognize antigen without preprocessing. The T cell cannot recognize antigen in its natural state. It must first be broken down and the fragments bound to a Major Histocompatibility Complex (MHC) molecule by the APC. The macrophage is an APC. Glucan causes its receptor sites (key slots) to be released for these presentation chores.

The killer T cells (trained assassins) now begin shooting holes in cells that have been infected by viruses or other pathogens. The killer T cell (CTL or cytotoxic T lymphocyte) becomes a "trained assassin". They respond to the MHC I complex, which is found on almost all body cells. The CTL has the ability to seek and destroy infected human cells in a specific manner. With the injection of powerful chemicals, these infected cells are killed before they can be used to spread a disease. Natural Killer (NK) cells are large, granule-filled lymphocytes that take on tumor cells and infected body cells. They are known as "natural" killers because they attack without first having to recognize specific antigens. Like the macrophage, if it is not "self", it will proceed to kill. NK cells and CTLs both kill on contact. The killer binds to the target, aims its weapons and then releases a lethal burst of chemicals to punch holes in the target. - source